A number of medical ailments may be treated and/or diagnosed through the application of a magnetic field to an afflicted portion of a human subject's body. Neurons and muscle cells are a form of biological circuitry that carry electrical signals and respond to electromagnetic stimuli. When a changing magnetic field is applied to a portion of the body, neurons may be depolarized and stimulated. Muscles associated with the stimulated neurons can contract as though the neurons were firing by normal causes.
A nerve cell or neuron can be stimulated in a number of ways, for example transcutaneously via transcranial magnetic stimulation (TMS). TMS typically uses a rapidly changing (e.g., pulsed) magnetic field to induce a current in a nerve cell, without having to cut or penetrate the skin, or apply electrodes. The nerve is said to “fire” when a membrane potential within the nerve rises above a threshold voltage with respect to its normal ambient level (e.g., approximately −90 millivolts, depending on the type of nerve and local pH of the surrounding tissue).
Magnetic stimulation has proven effective in stimulating regions of the brain, which is composed predominantly of neurological tissue. One area of particular interest is the treatment of depression. Repetitive transcranial magnetic stimulation (rTMS) has been shown to have significant anti-depressant effects for human subjects that do not respond to the traditional methods. A typical rTMS treatment may include application of a subconvulsive stimulation to an area of a subject's brain, for example the prefrontal cortex, in a repetitive, non-invasive manner. Such a treatment may cause a depolarization of cortical neuron membranes. The membranes may be depolarized, for example, by the induction of small electric fields in excess of 1 V/cm that may be generated by a rapidly changing magnetic field.
Typical TMS treatment apparatuses generate pulsed magnetic fields that induce currents in electrically sensitive cells (e.g., nerve cells or neurons). These induced currents typically form a complete circuit in the body, such that a path of zero current through the body is created. The currents induced by a TMS treatment apparatus typically drop off to zero in approximately the middle of this path. The rate of this current drop off may be slowed, for example by spreading the current density generated the TMS apparatus over a wide surface area. However, employing this approach may concentrate return currents, which may lead to higher rates of undesirable side effects (e.g., the stimulation of untargeted regions of a subject's brain).
A typical TMS treatment apparatus may include one or more electrically conductive stimulating coils. Such coils may be configured (e.g., wound) in a single layer, such that the coils may be disposed as close as possible to the tissue that is to be stimulated. Such coils may be capable of stimulating brain tissue at a desirable depth relative to the skull. However, the magnetic field, or fields, typically generated by such coils may cause an undesirably high level of surface stimulation (e.g., of nerves near the surface of the skull).